Counter cation-controlled air oxidation of manganese derivatives of tetrachlorocatechol

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Inorganic Chemistry Communications 3 (2000) 76–79

Counter cation-controlled air oxidation of manganese derivativesof tetrachlorocatechol

Rafael Ruiz a, Andrea Caneschi a, Dante Gatteschi a,*, Claudio Sangregorio a, Lorenzo Sorace a,Miguel Vazquez b´

a Dipartimento di Chimica, Universita di Firenze, 50144 Florence, Italy`b Departamento de Quımica Inorganica, Facultade de Quımica, Universidade de Santiago de Compostela, 15706 Santiago de Compostela, Spain´ ´

Received 13 December 1999

Abstract

The potassium salt of the triphenylphosphine oxide adduct of bis(tetrachlorocatecholate)manganese(III) complex reacts with O2 in thepresence of Kq-sequestering agents at room temperature in MeCN resulting in the formation of the corresponding MnIV-tris(catecholate)species. q2000 Published by Elsevier Science S.A. All rights reserved.

Keywords: Catecholato complexes; Hypervalent compounds; Manganese; O–O activation; Oxygenation; Potassium

Scheme 1.

Manganese catecholate chemistry is of great current inter-est in diverse areas ranging from redox chemistry andelectrontransfer to catechol oxidation and dioxygen reactivity [1,2].Catechol acting as a bidentate ligand through the strong elec-tron donating deprotonated oxygen atoms is able to stabilizetetravalent manganese [3–9]. Further, being a redox-activenon-innocent ligand, it can also give rise to several possibleredox isomers for which the formal oxidation states of themetal center are different (Scheme 1). In certain cases, intra-molecular electron transfer between the metal center and thecatechol ligand occurs giving rise to an equilibrium betweenmetal catecholate and semiquinone forms (resonance formsI and II, respectively) [6–10]. This new phenomenon,knownas valence tautomerism, has certainly further stimulated therecent interest in this field [11].

Likewise, manganese–catechol complexes can interactwith dioxygen either in a reversible manner forming Mn–O2

adducts [12,13], whose identity is still a matter of contro-versy [14,15], or through a wide variety of nonreversiblereaction pathways affording manganese-o-semiquinonecom-plexes [6,10], free benzoquinone [16,17], or even oxidativecarbon–carbon bond cleavage products of catechol [10,18].These studies have acquired an additional biological rele-vance since the discovery of manganese dioxygenase

* Corresponding author. Tel.: q39 055 354 841; fax: q39 055 354 845;e-mail: [email protected]

enzymes, a new class of extradiol dioxygenases with Mn(II)at the active site [19–22]. Much of the investigation in thisfield involved the oxygenation of Mn derivatives of 3,5-di-tert-butylcatechol (dtbcatH2), but little work has beendevoted to the dioxygen activation by manganese complexesof other catechols. We describe herein the formation of a newhigh-valent manganese(IV) complex of tetrachlorocatechol(Cl4catH2) from the reaction of lower-valent manganese pre-cursors with dioxygen.

Reaction of a manganese(II) salt with tetrachlorocatechol,prepared in situ from tetrachloro-1,2-benzoquinone and tri-phenylphosphine, in basic tetra-n-butylammonium hydrox-ide acetone/water solution under aerobic conditions yields

R. Ruiz et al. / Inorganic Chemistry Communications 3 (2000) 76–79 77


Fig. 1. Electronic absorption spectra of complexes 1 and 2 in acetonitrile.Inset: time course of the absorbance variation at 615 nm of CH3CN solutionsof 2 (0.5 mM) with or without 18-crown-6 (curves (a) and (b), respec-tively) in air at room temperature.

the manganese(IV) catecholate complex (Bu4N)2-[Mn(Cl4cat)3]PMe2CO (1) 1. The electronic spectrum ofthe dark blue complex 1 in acetonitrile (Fig. 1) exhibits anintense visible absorption band at 615 nm (´f7600 My1

cmy1), with two distinct shoulders at 685 and 425 nm, whichis typical of high-valent manganese(IV) catecholate species[3–9]. In fact, this band can be assigned to ligand-to-metalcharge-transfer (LMCT) transitions of type catecholato™MnIV on the basis of comparision to previously reportedtris(catecholate)manganese(IV) complexes with cat anddtbcat ligands [3–5]. Interestingly, under similar reactionconditions but with KOH in the place of Bu4NOH, Pierpontand co-workers obtained the manganese(III) catecholatecomplex K[Mn(Cl4cat)2(OPPh3)]PH2OPMe2CO (2)[23]. The structure of 2 consists of two bis(tetrachloro-catecholate)(triphenylphosphine oxide)manganese(III)complex anions, [Mn(Cl4cat)2(OPPh3)]y, which are linkedtogether by relatively weak out-of-plane manganese–catech-olate oxygen bonds. In addition, potassium cations are tightlybound to catecholate oxygen atoms from adjacent molecules(see Scheme 2), in a similar manner to that found in themanganese(IV)–tris(catecholate) complex K2[Mn(dtb-cat)3]P6MeCN [5]. The electronic spectrum of the lightgreen complex 2 in CH3CN (Fig. 1) shows only a weakvisible band at 635 nm (´s150 My1 cmy1) on the low-energy tail of an UV absorption band, which corresponds toa ligand field (d–d) transition of MnIII (d4, high spin).

In an attempt to understand this remarkable cation selec-tivity for the attainment of a particular oxidation state of themanganese ion in the corresponding Mn–catecholate com-plex, we have studied the oxygenation of 2 with potassiumcomplexing agents. Thus, the reaction of complex 2 with O2

1 Synthesis and selected spectral data for 1. A deaerated acetone solution(25 cm3) of Cl4catH2, prepared in situ by stoichiometric mixing of tetra-chloro-1,2-benzoquinone (0.50 g, 2.0 mmol) and triphenylphosphine (0.52g, 2.0 mmol), previously neutralized with a methanolic solution of Bu4NOH1.0 M (4.5 cm3, 4.5 mmol) was added dropwise under vigorous stirring inan inert atmosphere to a solution of MnCl2P4H2O (0.20 g, 1.0 mmol) indeaerated water (5 cm3). A light pink precipitate was formed initially (pre-sumably the neutral (tetrachlorocatecholate)manganese(II) complex), thatredissolved to give a light brown solution which was extremely air sensitive(it slowly became greenish-brown even when stirred under argon atmos-phere). The reaction mixture was further stirred under gentle reflux for 15min in air. The resulting dark green solution was filtered under vacuum toeliminate a red–brown insoluble solid (fundamentally MnO2), washed copi-ously with acetone, and reduced in volume on a rotatory evaporator to yielda dark green crystalline solid and a dark brown mother liquor. The darkbrown solution was separated from the dark green solid, which was thendissolved in acetone. The initial dark green acetone solution soon turnedgreenish-blue and became deep blue overnight, while a brown powderappeared in the bottom of the flask. Small dark blue crystals of complex 1not suitable for X-ray analysis were obtained from the filtered deep bluesolution after several days of slow evaporation in air at room temperature.They were filtered on paper and further recrystallized from acetone (30%).Satisfactory chemical analysis was obtained for C50H72Cl12MnN2O6PC3H6O(1335.6): calc.: C, 47.66; H, 5.89; N, 2.10; found: C, 47.70; H, 6.03; N,2.07%. nmax/cmy1 (KBr): 1716 (CO) from Me2CO, 1423 (CO) fromCl4cat. lmax/nm: 425 (sh) (´/My1 cmy1 3650), 615 (7600), 685 (sh)(6550) (MeCN).

in the presence of the Kq-specific chelator 18-crown-6 hasbeen followed by UV–Vis spectroscopy at room temperaturein CH3CN (inset Fig. 1, curve (a)). The appearance of theintense absorption band in the visible region (lmax 615 nm),associated with the blue color developed by the initial palegreen solution with time, is indicative of the formation of ahigh-valent manganese(IV) catecholate species. Indeed, theelectronic spectrum of the final deep blue solution is identicalto that of complex 1. In the absence of a potassium-chelatorthe oxidation of 2 proceeds to some extent, but with a dra-matically lower reaction rate (inset Fig. 1, curve (b)). Infact, complex 2 is rather stable in acetonitrile solution at roomtemperature, but the addition of excess tetraphenylphosphon-ium chloride (a general alkali metal chelator) produces animmediate color change from pale green to greenish-blue, toultimately yield a deep blue solution after a period of severalhours in air. From the final deep blue solution, we haveisolated the alkali metal free manganese(IV) catecholatecomplex (Ph4P)2[Mn(Cl4cat)3] (3) 2, and its structure hasbeen determined through single-crystal X-ray analysis 3.

2 Synthesis and selected spectral data for 3. To a solution of complex 2(0.94 g, 1.0 mmol) in MeCN (25 cm3) was added solid PPh4Cl (0.56 g,1.5 mmol) under stirring at room temperature. The initial pale green solutionsoon turned greenish-blue and became deep blue after several hours in air.Dark blue crystals of complex 3, contaminated with a small amount of red-brick powder, were deposited from the final deep blue solution overnight.Recrystallization from MeCN yields small dark blue hexagonal prisms of 3suitable for X-ray analysis after a few days of slow evaporation at roomtemperature. They were filtered on paper and air-dried (60%). Satisfactorychemical analysis was obtained for C66H40Cl12MnP2O6 (1471.3): calc.: C,53.88; H, 2.74; found: C, 53.69; H, 2.73%. nmax/cmy1 (KBr): 1423vs (CO)from Cl4cat. lmax/nm: 425 (sh) (´/My1 cmy1 3650), 615 (7600), 685(sh) (6550) (MeCN).

3 Crystal data for 3: C66H40Cl12MnP2O6, Ms1471.26, monoclinic, spacegroup C2/c, as23.273(5), bs14.152(5), cs20.138(5) A, bs˚

95.790(5)8, Us6598.8(32) A3, Ts293 K, Ds1.481 g cmy3, Zs4,˚

m(Mo-Ka)s0.785 mmy1, 4037 unique reflections, 2040 assumed as

R. Ruiz et al. / Inorganic Chemistry Communications 3 (2000) 76–7978


Scheme 2.

Fig. 2. Perspective view of the anionic mononuclear unit of 3 with the atom-numbering scheme (thermal ellipsoids are drawn at the 30% probabilitylevel). Selected bond distances (A) and angles (8) with standard devia-˚

tions in parentheses: Mn(1)–O(11) 1.892(6), Mn(1)–O(12) 1.885(5),Mn(1)–O(21) 1.877(6); O(11)–Mn(1)–O(12) 85.6(2), O(11)–Mn(1)–O(21) 90.8(2), O(11)–Mn(1)–O(11I) 92.9(4), O(11)–Mn(1)–O(12I) 90.8(2), O(11)–Mn(1)–O(21I) 175.1(3), O(12)–Mn(1)–O(21)92.7(2), O(12)–Mn(1)–O(12I) 174.8(3), O(12)–Mn(1)–O(21I)91.2(2), O(21)–Mn(1)–O(21I) 85.7(3) (symmetry code: Isyx, y,yzq1/2).

The structure of 3 consists of discrete mononuclear pro-peller-like tris(tetrachlorocatecholate)manganese(IV)com-plex anions, [Mn(Cl4cat)3]

2y, with a two-fold symmetry(Fig. 2), and tetraphenylphosphonium cations. The manga-nese atom is coordinated to six oxygen atoms from the biden-tate catecholate ligands in a trigonally distorted octahedralgeometry. The Mn–O bond lengths are nearly identical(aver-age bond distance of 1.88 A), and they are comparable to˚

those found for the other two structurally characterizedmanganese(IV)–tris(3,5-di-tert-butylcatecholate) com-plexes [4,5]. The O–Mn–O bond angles deviate from an idealoctahedron, owing to the constraints imposed by the five-membered chelate rings (average bite angle of 85.68), result-ing in the observed trigonal distortion of the manganesecoordination sphere (average twist angle of 53.98). Allstructural features are consistent with a manganese(IV)–catecholate formulation for 3 (i.e., the absence of Jahn–Tellerdistortion typical of MnIII for an alternative manganese(III)–semiquinone charge distribution). As a matter of fact, thetwo independent catecholate moieties have almost equivalentC–O and C–C bond lengths within the 3s criterion (averagebond distances of 1.32 and 1.39 A, respectively), which are˚

in the range observed for other manganese catecholate com-plexes [4–10,23].

In summary, we have found a novel alkali metal chelatingagent-promoted aerial oxidation of a manganese(III) dimerof tetrachlorocatechol to a high-valent manganese(IV)catecholate monomer (Scheme 2). As far as we are aware,there is only one precedent in the literature of this kind ofreaction by O’Halloran and co-workers who reported the airoxidation of a Mn(III)–bis(amidoalkoxo) dimer to aMn(V)–oxo complex [24]. It appears that loss of Kq bridges

observed with I)2s(I) which were used in all calculations. The structurewas solved using direct methods with subsequent full-matrix least-squaresmethod refinement on F2. The hydrogen atoms were calculated at fixeddistances and refined with an overall isotropic thermal parameter. Refine-ment of 393 variables with anisotropic thermal parameters for all non-hydrogen atoms gave Rs0.063 and Rws0.150 with Ss1.123.

in 2 leads to formation of a O2-reactive Mn species. Althoughthe exact nature of this intermediate species is unknown, amonomeric five-coordinate MnIII–bis(catecholate) triphen-ylphosphine oxide adduct or a dissociation product of 2 suchas MnIII–tris(catecholate) seem to us the more probable can-didates to interact with O2. An investigation of the mecha-nistic details of the air oxidation reaction is planned in orderto provide further insight into the process of dioxygen acti-vation by this new family of manganese tetrachlorocatecho-late complexes.

R. Ruiz et al. / Inorganic Chemistry Communications 3 (2000) 76–79 79


Supplementary material

Tables of atomic coordinates, bond lengths and angles, andthermal parameters are available from the authors on request.

Acknowledgements

This work was supported by the 3MD-EU Network (con-tract no. ERB 4061, PL 97-0197). We would like to expressour gratitude to Prof. A. Dei for fruitful discussions andcontinuous interest in this work.

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Counter cation-controlled air oxidation of manganese derivatives of tetrachlorocatechol

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